Problem: Solve for $a$ : $-30 = -28 + a$
Solution: Add $28$ to both sides: $ \begin{eqnarray} \\ \hphantom{-30 {+ 28}} &\hphantom{=}& \hphantom{a} \\ -30 &=& -28 + a \\ \\ {+28} && {+28} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -30 {+ 28} &=& a \\ \end{eqnarray} $ Simplify. $ a = -2$